The field of the present invention is stereolithographic apparatus for the production of three-dimensional objects.
Stereolithography has come to be employed as a method for the fabrication of complex prototype parts. This is accomplished by successively printing cross-sectional layers of the intended part on top of one another through the curing of a photocurable polymer at the surface of a vat of such material. After the curing of one cross-sectional layer, that cross section is lowered in the vat below the surface of the liquid by the desired thickness of the successive cross section. One or more of various methods are used to insure the formation of a smooth liquid coating over the just cured cross-sectional layer. The thickness of the smooth coating is equal to the desired thickness of the successive cross section. The successive cross section is then produced at the surface of the vat such that it is physically joined with the prior cross section. Through repetition of this process and probable changes in the successive cross sections, a physical part may be constructed. This is extremely useful in the making of prototype parts and the like. Stereolithographic systems are described in U.S. Pat. No. 4,575,330 which is incorporated herein by reference as if fully set forth.
Several means have been contemplated for the lithographic step. Electromagnetic radiation beams tracing across the target surface of photocurable polymer, masks with generally-directed radiation, chemical jets and heat form some of the possibilities which may be used to create the desired cross sections. One such system which has found substantial practical utility employs an electromagnetic beam typically generated by a laser. Computer-controlled dynamic mirror systems may be used with such a beam generator to generate a selected tracing of the beam at a photosensitive surface. In stereolithographic systems, the selected beam tracing generated is on the surface of a photocurable polymer liquid where each successive cross section is formed. Reference is made to U.S. patent application Ser. No. 07/331,644, filed Mar. 31, 1989, entitled "METHOD AND APPARATUS FOR PRODUCTION OF HIGH RESOLUTION THREE-DIMENSIONAL OBJECTS BY" the disclosure of which is incorporated herein by reference as if fully set forth.
There are a number of considerations which are advantageously undertaken in the design of a stereolithographic apparatus. When using a dynamic mirror system to trace the desired pattern, a laser beam may be used which is passed through a converging lens before being directed by the mirrors to the working surface. This lens is chosen to bring the beam to a focus on the working surface of curable material which is often a liquid photocurable resin. Before passing through this converging lens, the beam may be passed through a diverging lens in order to increase its size and thereby allow a smaller image point to be formed after being focused by the converging lens. All parts of the horizontal liquid surface upon which the beam is to be traced do not have the same path length from the dynamic mirror system; and, therefore, the beam may not be in optimum focus at all such parts of the horizontal liquid surface. The beam must be focused to a relatively fine point so that maximum resolution of details may be realized in the part being formed. Because of this focusing problem, a system employing a small field of view relative to the beam path length is desirable. This small field of view refers to the target surface dimensions (maximum width) being small relative to the part length between the scanning mirrors and the target surface. In other words, the angular displacement of the scanning mirrors should be small when traversing between extremes on the target surface. This design criteria is at odds with the need to make relatively large parts with such a system and keep the size of the system within reasonable limits.
The orientation of the beam is also of importance. As cure rates are affected by beam intensity (power/per unit area), it is advantageous to have a relatively uniform orientation of the beam on the surface. Similarly, a problem can occur whenever solidifying radiation impinges on the target surface at angles other than ninety degrees. When this happens, resin will be cured at these same angles, giving rise to a roughness of part finish known as the shingle effect. Therefore, a design consideration is to have the beam as close to perpendicular to the liquid surface as possible. Again, small patterns relative to the length of beam path enhance this desired condition.
It should be noted that the shingle effect is reduced by building layers which are thin relative to the error which can be tolerated. This is because it is the displacement in the X-Y position of the beam at the liquid surface and the X-Y position of the beam at one layer thickness below the surface that gives rise to the error which causes the shingle effect. The thinner the layers, the more off perpendicular the beam can strike the surface without producing significant shingling. In equation form, the maximum angular displacement of the beam from off center, .THETA., is equal to the arctangent of the error which can be tolerated divided by the layer thickness. For an error tolerance of 2 mils and a layer thickness of 20 mils, for example, the maximum angular displacement of the beam can be about 5.7 degrees. However, if the layer thickness is reduced to 5 mils, the maximum angular displacement can be increased to about 21 degrees.
Additional lenses and other optical devices can reduce or eliminate some or all of these problems but result in a substantial increase in the cost of the overall system.
Other design considerations influence the layout of the system. It is advantageous to have a large numerical aperture for the beam being focused on the working surface for the purpose of obtaining a sharp image from reduced beam diffraction. However, this advantage may be offset due to the much greater difficulty in maintaining a sharp focus over a wide working area because of both the smaller image and the wider incident and exit cone of the beam. Again, this makes a reduced field of view relative to the length of the beam path advantageous. Also affecting the design are the dynamic mirrors. They often present the limiting aperture in such a system. Mirror speed is affected by size since larger mirrors have higher inertia. The ability of mirrors to function over large reflective angles is also limited. At the same time, mirror control is more precise if a given increment of distance at the working surface requires a relatively large pivotal displacement of the mirror. Thus, the foregoing design considerations give conflicting requirements between long and short beam path lengths. A compromise "best distance" is preferably selected.
Thus, in designing stereolithographic systems, compromises between drawing speed, accuracy and system cost must be considered. Balancing these factors, it has been found that a path length in the range of about 2 to 5, and preferably in the range of about 3 to 4 times the maximum linear dimension of the working surface is a reasonable compromise of competing factors. This relationship may also be understood as having a beam intersection with the target surface not exceeding about 14 degrees from some nominal angle of incidence, such as, for example, perpendicular to the liquid surface. However, to obtain a large prototype part, these relationships then require a comparatively large stereolithographic system.
As mentioned earlier, the ratio between path length and maximum linear dimension of the working surface depends on several competing factors, and the preferable ratio for a particular situation depends on which of these factors may be dominant. As defined here, the path length is the distance the beam travels after it leaves the dynamic mirrors until it reaches the resin surface. The factors that yield the lower limit on the ratio range are: (1) shingling effects, (2) variation in beam intensity, (3) maximum angular speed and acceleration of the scanning mirrors, (4) difference in path length, etc. Depending on the significance and control over these factors, this lower limit may be increased or decreased. Similarly the factors that lead to the upper limit on the range are: (1) resolution of the angular placement of the scanning mirrors, (2) size of the scanning mirrors necessary to get a particular spot size at the target surface, etc. Depending on the significance and control over these factors, this upper limit may be increased or decreased.
A path length which is too long may have the following negative consequences. First, since the linear resolution associated with tracing the beam on the surface is equal to the angular resolution multiplied by the path length, a long path length may result in an unacceptable loss of linear resolution. Second, a longer path length may require larger dynamic mirrors in order to achieve a desired beam size at the resin surface. This is because the size of the dynamic mirrors may be the limiting aperture in the optical system. The dynamic mirrors, however, are typically very expensive, larger ones even more so. In addition, larger dynamic mirrors may not have the speed or acceleration of smaller mirrors.
A path length which is too short, on the other hand, will also have negative consequences. First, a shorter path length will exacerbate the shingling effect, perhaps unacceptably, since the beam will strike the resin surface, especially at the outer extremes of the vat, at more exaggerated angles. Second, a shorter path length may result in a non-uniform intensity at the resin surface, especially at the outer reaches of the vat, where the angle of incidence is more extreme. This may result in a non-uniform cure depth. Third, a shorter path length may result in a more significant difference between the path lengths of a beam directed to the center of the vat, and one directed to the outer extremes of the vat. Therefore, at the outer reaches, the beam may intersect the resin surface outside the range of the beam waist. The beam waist is the focal area around the point where the vertices of the incident and exit cones of the beam touch, such that the beam size is approximately constant. The beam will therefore be out of focus in the outer reaches, so the beam size will be too large. This can also result in a non-uniform cure depth due to an associated decrease in beam intensity. An additional difficulty arises from the need to vary the angular scanning speed of the mirrors as the path length changes in order to maintain constant translational speed of the beam on the target surface, which is desirable in order to maintain uniform exposure.
In sum, for the reasons set forth above, it has been found that for the present embodiment, a path length which is about in the range of 2 to 5, and preferably, in the range of about 3 to 4 times the maximum linear dimension of the target surface will be an acceptable range. Of course, for other embodiments, a different range may be appropriate, since one or more of the factors mentioned above may become more significant. The subject invention is intended to encompass these other embodiments, and also corresponding changes in the ratio of path length to maximum linear dimension of the target surface.
By expanding the size of the machine, practical difficulties inherently exist. The size itself becomes impractical. Since in the preferred embodiment, a free liquid surface is used, and it is horizontal, the beam is to be directed vertically from above the working surface. These same difficulties arise in embodiments where exposure occurs through the bottom of the vat containing the liquid resin, or through the side of the vat. In these cases, increasing the working surface can be limited by system length. Increasing the target area can, therefore, be limited by system height. The structures required in maintaining the relative positions of all components can also be complicated or require greater attention in manufacture and use. Aiming of the beam is made more complicated. Maintaining a coherent beam can be more complicated as well.
Proper control of the beam in such stereolithographic systems has been addressed. Reference is again made to U.S. patent application Ser. No. 07/331,644, and to U.S. patent application Ser. No. 07/428,492, which is fully incorporated by reference herein as though set forth in full. The beam profile itself may be measured such that the beam power and focus may be controlled and the cure depths and width of plastic created in the photocurable liquid may be predicted. Reference is made to U.S. Pat. No. 5,058,988, and to U.S. patent application Ser. No. 07/429,911, the disclosures of which are incorporated by reference herein as if fully set forth. Correcting the drift of a beam due to any cause has also been addressed. Reference is made to U.S. Pat. No. 5,059,021, the disclosure of which is incorporated herein by reference as if fully set forth. Further, calibration and normalization of the beam across a grid on the target surface has also been addressed in U.S. patent application Ser. No. 07/268,837, the disclosure of which is incorporated herein by reference as if fully set forth. Each of these processes and the apparatus therefor are designed to correct the beam at or about the target surface. Thus, they cooperate with the system computer control regardless of the equipment employed to generate and deliver the conditioned beam to the target surface.
Other teachings relevant to stereolithographic systems for creating three-dimensional parts are found in the following disclosures which are incorporated herein by reference as if fully set forth: U.S. Pat. Nos. 4,999,143 and 5,015,424, U.S. patent application Ser. Nos. 07/268,429 and 07/339,246 and International Patent Application serial no. PCT/US89/4096.